The present invention relates generally to computed tomography (CT) imaging systems. More particularly, it relates to a method and apparatus for heating the detector rail of a CT imaging system.
In at least some CT imaging system configurations, a stationary floor-mounted frame includes an x-ray source and a radiation detector array. The x-ray source projects a fan-shaped beam that is collimated to lie within an X-Y plane of a Cartesian coordinate system and is generally referred to as the “imaging plane”. The x-ray beam passes through the object being imaged, such as a patient. The beam, after being attenuated by the object, impinges upon the array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is dependent upon the attenuation of the x-ray beam of the object. Each detector element of the array produces a separate electrical signal that is a measurement of the beam attenuation at the detector location. The attenuation measurements from all the detectors are acquired separately to produce a transmission profile. The x-ray source and the detector array are rotated with a gantry within the imaging plane and around the object to be imaged. The X-ray source typically includes an x-ray tube that emits an x-ray beam. The X-ray detectors typically include a collimator for collimating x-ray beams received at the detector. A scintillator is located adjacent the collimator and photodiodes are positioned adjacent the scintillator.
In CT imaging, the gantry is used to rotate the x-ray source and the detector array around an object to be imaged so that the data corresponding to every angle is collected. Thereafter, the collected data is filtered, weighted, and typically is back projected by an image process to generate one or more diagnostic quality images.
In image reconstruction, it is assumed that the gain of each detector remains constant throughout a data acquisition process and that any change in x-ray signal intensity at the detector is due to patient anatomy. Unfortunately, this is not the case for several reasons. One particularly acute source of error in this regard has to do with how detector elements are affected by ambient conditions during operation. More specifically, detector element response to x-ray intensity varies as a function of temperature.
Also, temperature gradients along array rails and between rails have been known to change the relative positions of the rails. Even slight variations in the temperature of the detector rails can cause this misalignment. Obviously, if the detector elements are slightly misaligned, the resulting image will also be inaccurate.
There are other detector array components affected by changes of temperature. Specifically, the shunt resistance of a photo diode drops exponentially with temperature which results in leakage current and in general a decrease in the signal noise to ratio.
When array output varies as a function of element and array environment temperature, the quality of the resulting image is adversely affected. To this end, it has been observed that temperature effects on array output sometimes result in image artifacts that adversely affect the diagnostic usefulness of the resulting images.
There are many sources of heat in CT systems that directly affect the temperature of the array. Specifically, the x-ray tube used to generate the x-ray beam generates a large amount of heat in CT systems. In recent years, the problem has become more acute because of the desire to increase patient through-put. This has fueled the use of more powerful x-ray sources such that the amount of data required to generate images can be acquired in a shorter period of time. These high powered systems, while appreciably faster than their predecessors, have exacerbated the array heating problem and thus the associated image degradation.
In order to address temperature related array operation problems, prior devices have provided several heating systems to keep the array at an elevated, constant temperature. These heating systems are generally used to heat the elements to an expected temperature level and to maintain the temperature level throughout an acquisition. The CT imaging machine is then calibrated for optimum image of quality at the expected temperature.
Unfortunately, the array temperatures occurring in high power systems often exceed the temperature bound which renders the heating configurations ineffective in maintaining an isothermal condition. More simply, when detector temperature exceeds a target expected temperature level during some portion of an acquisition, the heating configuration which is limited by the upper temperature range is effectively useless.
Therefore, there remains a need for a simple and economic method for maintaining a detector array at a constant temperature, especially in conjunction with high-powered x-ray tubes.